Little Bullies of the Sky

Wherein a neutron star invades, destroys, and rebuilds its neighbor in slavish orbit, betraying itself only by a wisp of lithium.

Astronomers aren’t easily surprised by things astronomical. They take black holes, curved space, quasars, and many other wonders in stride. Yet Philip Charles of Oxford University had occasion to be nonplussed during a recent astronomy meeting. He had just finished giving a talk on a fairly unspectacular subject--the lithium content of certain X-ray-emitting star pairs--when he was buttonholed by Philipp Podsiadlowski of Cambridge University. Podsiadlowski pointed out that Charles may inadvertently have found the first evidence that a superdense neutron star can plunge to the core of a giant neighbor, disrupting that star’s nuclear furnace and causing its vast atmosphere to drift off into space, where the gas would clump together again into a new and smaller star in orbit around the neutron-star invader. The lithium, said Podsiadlowski, was the smoking gun.

As it turns out, he wasn’t just daydreaming, and the weird scenario he described wasn’t entirely his invention. Some 20 years ago astrophysicists Kip Thorne of Caltech and Anna Zytkow of Cambridge proposed that a neutron star that started out in orbit around a red giant might eventually sink to the giant’s core. Although a neutron star is only ten miles or so across, its gravitational pull is so strong that it could draw matter away from its huge but more diffuse companion. In the process, it would slow down, like a ship dragging an anchor. Eventually the neutron star, in an ever-shrinking orbit, would plow into the outer layers of its neighbor. After a few thousand years it would spiral down into the star’s center, demolishing the existing stellar core but leaving the rest of the star essentially intact. A red giant that has been violated in this way has come to be called a Thorne-Zytkow object--even though no one has ever seen one.

Charles wasn’t looking for one, either. Indeed, he was studying objects that are in some sense the opposite of Thorne-Zytkows: X-ray binaries. Instead of a neutron star orbiting a red giant, an X-ray binary is believed to consist of a small (sun-size) star orbiting either a neutron star or that other species of massive invisible body, a black hole. As the massive object pulls material off its small companion, the material gets so hot that it emits bursts of X-rays. I was studying these systems because they contain our best black-hole candidates, says Charles.

While looking at the spectra of several X-ray binaries, Charles found that they all contained unusually high amounts of the element lithium. This was unexpected: lithium is usually destroyed in nuclear reactions deep within a star, and so the characteristic wavelength of light it emits doesn’t show up in the star’s spectrum. We would have expected to find none, says Charles.

But the presence of lithium made immediate sense to Podsiadlowski, a theoretician who has thought a lot about Thorne-Zytkow objects. He thinks what Charles has been studying are not the T-Z objects themselves but star systems that are descended from them. In fact, he thinks all X-ray binaries may arise in that way.

Once a neutron star sinks to the center of a red giant to form a Thorne-Zytkow object, says Podsiadlowski, the physics of the star changes dramatically. Most stars have a large, hot core where energy-producing fusion reactions occur. Above this burning zone is a convective layer where hot gases rise, cool, and then sink back toward the core. A Thorne-Zytkow object, however, has at its center a ten-mile-wide neutron star rather than a core thousands of miles across. Nuclear fusion still occurs in the hybrid--matter falling into the neutron star gets compressed and heated enough--but only in a thin shell around the neutron star, at the bottom of the convective layer.

According to Podsiadlowski, the different locus of burning in a Thorne-Zytkow object explains why Charles saw his lithium. Although lithium is a normal by-product of fusion reactions in stars, in most stars high- speed protons in the core slam into the lithium atoms and transform them into beryllium (which later breaks down into helium). But in a Thorne- Zytkow object, Podsiadlowski says, fusion actually takes place in the bottom layer of the convective zone, which allows some lithium to escape before it can be converted into beryllium. Rising gases carry it to the outer layers of the star, where it becomes visible.

Eventually the feeble fusion reactor inside a Thorne-Zytkow object fails altogether, and the hybrid can no longer withstand its own gravity. The outer envelope of the star collapses like a pricked balloon, and part of it forms a lithium-enriched disk around the neutron star. Some of the matter in the disk, says Podsiadlowski, might coalesce into a second-generation star in orbit around the neutron star, much the way planets coalesced around our sun. The born-again star might then begin losing matter to its dense neighbor again--resulting in just the sort of X- ray-emitting, lithium-rich binary that Charles observed.

One great virtue of Podsiadlowski’s theory, convoluted and outlandish as it sounds, is that it offers an explanation of where X-ray binaries come from. Astronomers have had a hard time explaining how the small companion in an X-ray binary could have survived while its larger neighbor exploded as a supernova, which is the only way to make a neutron star. (A neutron star is the dense remnant of the exploded star’s core.) In Podsiadlowski’s scenario, the companion starts out as a red giant so large it can withstand a nearby supernova explosion. Only when it is later invaded by the neutron star does the giant die, to be reborn later in smaller form.

Charles himself does not buy Podsiadlowski’s theory. He has a less colorful explanation for the excess lithium: some of the X-rays emitted by matter falling onto the neutron star, he says, strike incoming carbon and nitrogen atoms, splitting them into lighter elements, including lithium. But Charles admits that this explanation ignores the question of how the damned things--the X-ray binaries, that is--formed in the first place.

Thorne-Zytkow objects should forge a variety of strange elements besides lithium, and if Podsiadlowski is right, astronomers might be able to detect those other elements in X-ray binaries too. Until then Podsiadlowski may have trouble making a lot of converts to his theory. It’s certainly speculative, and many astronomers may find it too speculative or exotic, he says. However, what you see is often more exotic than anything you’ve ever thought about.